dap_to_swd() function
Put the debug link into SWD mode, if the target supports it. The link's initial mode may be either JTAG (for example, with SWJ-DP after reset) or SWD. Note that targets using the JTAG-DP do not support SWD, and that some targets which could otherwise support it may have been configured to disable SWD signaling
Return value
ERROR_OK or else a fault code.